1. Field of the Invention
The present invention relates to a frequency generator and, more particularly, to a ring oscillator in which the current value is externally controlled in order to control the period of frequency, to thereby finely control the frequency and make it possible to use in various frequency bands.
2. Discussion of Related Art
Recently, a circuit configuration which can be used in various operation frequency bands has been required in electronic systems. That is, the operation frequency range of circuits becomes wider. As shown in FIG. 1, a conventional ring oscillator generates a frequency using a signal transmission delay value obtained by one NAND gate and N inverter circuits.
Referring to FIG. 1, the conventional ring oscillator is constructed in such a manner that one NAND gate having two inputs, a plurality of inverters I.sub.1, I.sub.2, . . . , I.sub.N-1, I.sub.N for delaying the output signal of the NAND gate are serially connected, the output signal from inverter I.sub.N is applied to one input of the NAND gate, and a control signal is applied to the other input of the NAND gate. By doing so, a delay signal of the transmitted signal obtained through the inverters is fedback to the input of the NAND gate, and the delay signal and a control signal which is applied from a main circuit are logically operated, to thereby generate a predetermined frequency.
In order to generate a desired frequency, the number of inverters is increased or decreased in the aforementioned conventional ring oscillator. That is, the hardware of the ring oscillator should be changed. As described above, in case that the frequency should be changed, or a frequency in a different band is required, it is impossible to externally operate the conventional ring oscillator. Furthermore, a new oscillator must be fabricated for the purpose of changing its hardware.